1. Field of the Invention
This invention relates to rotatable shafts and more particularly to means for electrically connecting a stationary electrical source to the interior region of a hollow shaft.
2. Description of the Prior Art
Large steam turbines used in central power station applications must be periodically shut down for maintenance and other reasons such as refueling a nuclear power reactor. Shaft rotation of large turbines is continued after shutdown at low speeds by utilizing turning gears. When it is desired to bring the turbine up to synchronous speed and load, the metal in the turbine's stationary and rotatable parts must be increased in temperature at a relatively slow rate to avoid formation of high internal stresses which can result in mechanical failure. To avoid such overstressing and possibility of mechanical failure, the metal temperature of large steam turbines is raised slowly by gradually increasing both the steam temperature and mass flow rate therethrough. Such gradual increases result in inordinately long elapsed times between turbine shutdown or turning gear operation and full load, synchronous speed operation.
Such long elapsed times are very expensive and reduce the turbine's availability to meet rapidly increasing load demands.
To compensate for such slow starting capability additional, partially loaded turbines may be continuously operated so as to obviate the need for fast startups of shut-down turbines. However, additional power generation equipment is expensive and may have to be operated at offdesign, lower efficiency conditions to satisfactorily respond to load demand variations. Thus, additional generating capacity could result in not only higher capital costs, but also presents the possibility of higher operating costs. It would also be desirable to have a fast starting steam turbine which did not require maintaining a heating fluid at a temperature level above the turbine metal transition temperature. Shaft metal near the shaft's bore is the highest stressed and the thermally slowest responding turbine portion to steam heating on the shaft's exterior.
A possible solution to the aforementioned problems is the use of electric bore heaters inserted in the hollow turbine shaft's interior. Such placement of electrical heaters requires an electrical conducting arrangement which transfers electricity from an exterior source to the shaft's interior while insulating the shaft from such electrical conduction. Devices used in prior applications for receiving electricity from a stationary source and transferring it to a rotating member include slip rings which were formerly utilized on generators before the advent of brushless exciters. Such slip rings were seldom larger than 14 or 15 inches in diameter because they were not required for generators of relatively low ratings and the contrifugal force acting on rings larger than that was excessive and frequently resulted in ring failure. Additionally, the contacting brushes for the slip rings had relatively short lives for larger diameter slip rings since surface speeds of such larger slip rings increased as the slip ring diameter increased.
It has been found to be highly desirable to mount the slip rings on the shaft's driving flange between the steam turbine and the generator since such arrangement provides easy access to the contacting brushes and obviates the need for a longer turbine and turbine rotor. The driving flanges on a turbine rotor are frequently 30 inches in diameter and more. Use of radial leads to transfer electrical energy from the shaft's outer periphery to its interior bore is suggested, but introduces the further problems of connecting it with the peripherally disposed slip ring. A mechanical connection between the slip ring and radial conductor often requires some kind of opening to be formed radially through the slip ring to provide later insertion of the radial conductor therein. Such opening formation in the already highly stressed slip ring further reduces the slip ring's strength and augments stress concentration effects. Metallurgical joints formed by processes such as brazing between the slip ring and the radial conductor have disadvantages in that localized high temperatures inherent in such processes may damage the insulation between such conducting arrangement and the shaft.
The aforementioned problems could be alleviated by apparatus which permits rapid steam turbine startup from turn gear operation to full load and, at the same time, increases the reliability of the turbine over presently acceptable designs.